Structural evidence for induced fit and a mechanism for sugar/H+ symport in LacY

Abstract

Cation‐coupled active transport is an essential cellular process found ubiquitously in all living organisms. Here, we present two novel ligand‐free X‐ray structures of the lactose permease (LacY) of Escherichia coli determined at acidic and neutral pH, and propose a model for the mechanism of coupling between lactose and H+ translocation. No sugar‐binding site is observed in the absence of ligand, and deprotonation of the key residue Glu269 is associated with ligand binding. Thus, substrate induces formation of the sugar‐binding site, as well as the initial step in H+ transduction.